High Electron Mobility Transistors (HEMTs), also called heterostructure field-effect transistors (HFETs) or modulation-doped field-effect transistors (MODFETs), are field effect transistors typically incorporating a junction between two materials with different band gaps, e.g., a heterojunction, as the channel instead of a doped region. HEMTs use high mobility electrons generated by a heterojunction comprised of a highly-doped wider-bandgap n-type donor-supply layer, or unintentionally doped Aluminum-Gallium-Nitride (AlGaN), for example, and a non-doped narrower-bandgap layer with little or no intentional dopants, e.g., Gallium-Nitride (GaN).
For example, electrons generated in an n-type donor-supply layer can drop into the non-doped narrower-bandgap channel at the heterojunction to form a thin depleted n-type donor-supply sub-layer and narrower-bandgap channel, due to the heterojunction created by different band-gap materials forming an electron potential well in the conduction band on the non-doped side of the heterojunction. In the framework of AlGaN/GaN hetero structures, there is often no dopant required in the AlGaN layer due to the strong spontaneous and piezoelectric polarization effect in such systems. For example, electrons from surface donors can be swept into the GaN channel by the intrinsic polarization induced electric field. In this instance, the electrons can move quickly without colliding with any impurities, due to the unintentionally doped (e.g., not intentionally doped) layer's relative lack of impurities or dopants, from which the electrons cannot escape. The net result of such a heterojunction is to create a very thin layer of highly mobile conducting electrons with very high concentration or density, giving the channel very low resistivity. This layer is known as a two-dimensional electron gas (2DEG). As can be expected in field-effect transistors (FET), voltage applied to the gate alters the conductivity of this layer to form transistor structures.
One kind of high-electron mobility transistor (HEMT) including Gallium Nitride is known as an Aluminum Gallium Nitride/Gallium Nitride (AlGaN/GaN) HEMT, or an AlGaN/GaN HEMT. Typically, AlGaN/GaN HEMTs can be fabricated by growing crystalline films of GaN, AlGaN, etc. on a substrate (e.g., sapphire, silicon (Si)(111), silicon carbide (SiC), etc.) through an epitaxial crystal growth method (e.g., metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), etc.) and processing the epitaxial substrate thus grown, to form the desired structures.
Group III-nitride (III-N) compound semiconductor materials, including GaN, advantageously possess a wide band gap (or bandgap), a high breakdown electric-field, a large thermal conductivity, and excellent semiconductor behavior at temperatures exceeding 250° C. In addition, a wide-bandgap heterostructure system, e.g., incorporating an AlGaN/GaN heterostructure enhanced by spontaneous and piezoelectric polarization effects, can yield a 2DEG channel with a high sheet charge concentration and high electron mobility, both of which lead to high current driving capability.
As such, Group III-nitride semiconductor materials, especially in the form of heterostructures, are favored candidates for fabricating power semiconductor devices. For example, III-nitride heterostructures can generate record output power densities at microwave and millimeter-wave frequencies. In addition, the III-nitride heterostructure devices, especially in the form of HEMT, are capable of delivering high operating frequency and high breakdown voltage simultaneously. Thus, the III-nitride heterostructure devices are also ideal candidates for the development of high-performance digital/analog mixed-signal integrated circuits.
AlGaN/GaN HEMTs are suitable for high-power, high-frequency, and high-temperature operations, because of the material advantages and high density carriers in the 2DEG channel. The conventional Ga-face AlGaN/GaN HEMT features strong spontaneous and piezoelectric polarization effect that results in very high 2DEG density in the range of approximately 1013 charges per square centimeter (cm2) even without any intentional doping, which is an order of magnitude higher than those obtained in gallium arsenide (GaAs) and indium phosphide (InP) based HEMT structures that must feature intentional doping. Although the high 2DEG density is beneficial to achieving high current density and consequently, low on-resistance, it also results in a HEMT device that requires a negative gate bias to turn off the conduction current, thus, presenting the conventional AlGaN/GaN HEMT devices as normally-on or depletion mode devices, which are defined as those with negative threshold voltage.
However, in circuit applications, normally-off or enhancement-mode HEMTs featuring positive threshold voltage are highly desirable because of simplified circuit configurations, reduced circuit complexity and simplified protection scheme. For instance, in radio-frequency (RF), microwave, and millimeter-wave circuits, normally-off devices enable the use of single-polarity (positive) supply voltage by eliminating the negative supply voltage. In GaN-based digital ICs, due to the lack of high-performance p-channel devices and low intrinsic hole mobility, CMOS-like implementation using p-channel and n-channel devices may not be a good choice. Instead, an all n-channel FET implementation strategy would be a needed for realizing high-performance (e.g., high speed, high voltage swing, etc.). The direct coupled FET logic (DCFL) featuring E-mode FET as the driver and D-mode FET as the load is the simplest configuration for n-channel FET logic circuits. In addition, in power electronics application, normally-off devices are highly desirable because of their inherent fail-safe operation as the current conduction is naturally shut off in case the gate control is lost.
Various conventional techniques for fabricating normally-off AlGaN/GaN HEMTs can include gate-recess by thinning down the gate barrier layer, fluorine plasma ion implantation by implanting fluorine ions into the gate barrier, use of thin gate barrier, and the use of p-type cap layer (e.g., GaN or AlGaN) or an InGaN cap layer. However, these structures typically require sophisticated fabrication process techniques such as, for example, low-damage dry etching, F (fluorine) plasma treatments, ion implantation, etc., which typically rely on the principle of depleting the 2DEG in the channel at zero gate bias.
Another challenge in the implementation of conventional AlGaN/GaN HEMTs is the relatively large off-state leakage current through the buffer layer, which could also result in premature breakdown in AlGaN/GaN HEMTs. For instance, in conventional HEMTs, both the source and drain are typically formed by making ohmic contact to the 2DEG channel. However, if the buffer is leaky, the off-state source-drain leakage current will be large, leading to undesirable features such as large off-state power consumption. Conventional undoped GaN buffer layers, unfortunately, usually feature non-negligible leakage due to the high background doping (e.g., by Si, (oxygen (O) impurities, etc.). Thus, sophisticated buffer techniques have been developed (e.g., AlGaN buffer, carbon (C) or iron (Fe) compensated doping in GaN) to suppress the leakage current that could otherwise result. It is thus desired to provide practical normally off HEMT devices with minimal off-state leakage current.